Oil and petrochemical companies have discovered vast quantities of natural gas in remote locations such as in polar regions and under seas. Transport of natural gas, which consists mostly of methane, is difficult and methane cannot presently be economically converted into more valuable products, such as hydrogen, or into products that are more economically contained or transported, such as liquid fuels including methanol, formaldehyde and olefins.
Typically, the methane is converted to synthesis gas (syngas), an intermediate in the conversion of methane to liquid fuels. Syngas is a mixture of hydrogen and carbon monoxide with a H.sub.2 /CO molar ratio of from about 0.6 to about 6.
The conversion of methane to syngas is presently accomplished by either a methane steam reforming process or a carbon dioxide reforming process. Methane steam reforming is an endothermic reaction: EQU CH.sub.4 +H.sub.2 OCO+3H.sub.2. (1)
This process has a relatively high yield of hydrogen gas (H.sub.2), producing three moles of hydrogen gas for each mole of carbon monoxide produced. The reaction kinetics require the addition of significant amounts of heat rendering the process economically less desirable.
Carbon dioxide reforming is also an endothermic process: EQU CH.sub.4 +CO.sub.2 2CO+2H.sub.2. (2)
The carbon dioxide reforming reaction is somewhat less efficient than the steam methane reforming reaction, generating one mole of hydrogen gas for every mole of carbon monoxide formed. The endothermic reaction requires the input of a significant amount of heat, rendering the process also economically less desirable.
Another approach is the direct partial oxidation of methane which can utilize an ionic conducting membrane reactor or a mixed conducting membrane reactor in accordance with the equation: EQU CH.sub.4 +1/2O.sub.2 CO+2H.sub.2. (3)
In an ionic or mixed conducting membrane reactor, a solid electrolyte membrane that has oxygen selectivity is disposed between an oxygen containing feed stream and an oxygen consuming, typically methane-containing, product stream. "Oxygen selectivity" means that oxygen ions are transported across the membrane while other elements, and ions thereof, are not. The solid electrolyte membrane is made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides, often having a fluorite or a perovskite structure.
At elevated temperatures, typically in excess of 500.degree. C., and preferably in the range of 700.degree. C.-1200.degree. C., the solid electrolyte membranes contain mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the material. Because the membranes allow only oxygen transport, they function as a membrane with an infinite selectivity for oxygen and are therefore very attractive for use in air separation processes.
In an ionic type system, the membrane transports only oxygen ions and the two electrons released by the oxygen in the course of equation (3) are transported across the membrane by an external electric field.
U.S. Pat. No. 4,793,904 to Mazanec et al., that is incorporated by reference in its entirety herein, discloses an ionic transport membrane coated on both sides with an electrically conductive layer. An oxygen-containing gas contacts one side of the membrane. Oxygen ions are transported through the membrane to the other side where the ions react with methane or similar hydrocarbons to form syngas. The electrons released by the oxygen ions flow from the conductive layer to external wires and may be utilized to generate electricity.
In a mixed conductor-type membrane, the membrane is a dual phase ceramic having the ability to selectively transport both oxygen ions and electrons. With this type membrane, it is not necessary to provide an external electric field for removal of the electrons released by the oxygen ions. U.S. Pat. No. 5,306,411 by Mazanec et al., that is incorporated by reference in its entirety herein, discloses application of a mixed conductor-type membrane. The membrane has two solid phases in a perovskite crystalline structure: a phase for oxygen ion transport and a second phase for electron conduction. The oxygen ion transport is disclosed as being useful to form syngas and to remediate flue gases such as NO.sub.x and SO.sub.x.
U.S. Pat. No. 5,573,737 to Balachandran et al. also discloses the use of an ionic or mixed conducting membrane to separate oxygen and subsequently react the oxygen ions with methane to form syngas.
The partial oxidation reaction is exothermic and once initiated does not require the additional input of heat. However, the yield of two moles of hydrogen gas per mole of carbon monoxide produced is 33% lower than the yield achieved by conventional steam methane reforming (see equation 1).
Integration of an ion transport membrane with other apparatus or processes to enhance either yield or efficiency is disclosed in commonly assigned U.S. patent application Ser. No. 08/848,200, now abandoned, entitled "Method of Producing Hydrogen Using Solid Electrolyte Membrane" by Gottzmann et al., filed on Apr. 29, 1997, and is incorporated by reference in its entirety herein. An oxygen selective ion transport membrane and a proton (hydrogen ion) selective membrane are combined to enhance hydrogen gas production. The oxygen ions transported through the oxygen selective membrane are reacted with hydrocarbons to form syngas. The syngas contacts a proton selective membrane that selectively transports hydrogen ions to be reformed as hydrogen gas.
Commonly owned U.S. patent application Ser. No. 08/848,258 now U.S. Pat. No. 5,865,878 entitled "Method for Producing Oxidized Product and Generating Power Using a Solid Electrolyte Membrane Integrated with a Gas Turbine" by Drnevich et al., filed on Apr. 29, 1997, and is incorporated by reference in its entirety herein, discloses the integration of an ion transport membrane with a gas turbine. An oxygen-containing gas stream contacts an oxygen selective ion transport membrane. Oxygen ions transported through the membrane are used to generate oxidized products. The oxygen depleted feedstock stream, that is heated during the exothermic reaction, is delivered to a gas turbine combustor at an elevated temperature.
Direct partial oxidation is also possible using oxygen that has been separated outside of the reactor, such as by distillation or pressure swing adsorption (PSA). A conventional catalytic chemical reactor can be used for the reaction and, in which instance, no membrane is necessary. Direct partial oxidation can also be done using air, rather than oxygen, however, the process becomes less economical.
While integration enhances the economic desirability of the direct partial oxidation reaction, there remains a need to enhance the yield achieved by the process to levels approximately equivalent to the steam methane reforming process.